1. Field of the Invention
The present invention relates to improvements in apparatus for low energy ion implantation.
2. Description of the Prior Art
Low energy ion implantation apparatus are known which are of the type disclosed in "Low Energy Ion Beam Formation for Film Formation," Proceedings of the Fifth Symposium on Ion Sources & Ion Asisted Technology, June 1981. With the apparatus of this type, the ions emitted from an ion source and passing through a mass separator are further passed through a deceleration tube and thereby decelerated. In this case, the energy of the ion beam reaching a disk having thereon the workpiece to be ion-implanted is given by the deceleration voltage of a deceleration power supply. Accordingly, the deceleration voltage, when set to a desired low level, affords an ion beam of low energy.
With such an ion implantation apparatus, however, ions of masses other than the desired mass impinge on the inner wall of the mass separator to produce a large amount of secondary electrons. The secondary electrons moving into the deceleration tube from the interior of the mass separator are subjected to a positive force within the deceleration tube and thereby acclerated in the direction of advance of the ion beam to form a stream of secondary electrons. This stream of secondary electrons strikes the disk with high energy emitted by the extraction power supply voltage minus the deceleration voltage (i.e., the voltage between the opposite ends of the deceleration tube). Consequently, the secondary electrons influence the measurement of the beam current of the ion beam. Moreover, the amount of secondary electrons varies with the course of the ion beam.
The influence exerted by the high velocity secondary electrons, that is, the high energy secondary electrons on the measurement of the beam current is slight in ion implantation apparatus having a medium or lower ion beam current (for example of up to about 1 mA). However the influence becomes pronounced with high current apparatus wherein the amount of secondary electrons is inevitably greater. Thus, in high current apparatus it becomes difficult to accurately measure the ion beam current. Further if it is impossible to accurately measure the ion beam current, difficulties will be encountered in adjusting the amount of ion beam current and in assuring the implantation of an accurate amount of ions based on the measurement.